Top quark

The top quark, also known as the t quark (symbol: t) or truth quark, is the most massive of all observed elementary particles. Like all quarks, the top quark is a fermion with spin 1/2, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. It has an electric charge of +2/3 e. It has a mass of 173.0 ± 0.4 GeV/c2,[1] which is about the same mass as an atom of rhenium.[2] The antiparticle of the top quark is the top antiquark (symbol: t, sometimes called antitop quark or simply antitop), which differs from it only in that some of its properties have equal magnitude but opposite sign.

The top quark interacts primarily by the strong interaction, but can only decay through the weak force. It decays to a W boson and either a bottom quark (most frequently), a strange quark, or, on the rarest of occasions, a down quark. The Standard Model predicts its mean lifetime to be roughly 5×10−25 s.[3] This is about a twentieth of the timescale for strong interactions, and therefore it does not form hadrons, giving physicists a unique opportunity to study a "bare" quark (all other quarks hadronize, meaning that they combine with other quarks to form hadrons, and can only be observed as such). Because it is so massive, the properties of the top quark allow predictions to be made of the mass of the Higgs boson under certain extensions of the Standard Model (see Mass and coupling to the Higgs boson below). As such, it is extensively studied as a means to discriminate between competing theories.

Its existence (and that of the bottom quark) was postulated in 1973 by Makoto Kobayashi and Toshihide Maskawa to explain the observed CP violations in kaon decay,[4] and was discovered in 1995 by the CDF[5] and [6] experiments at Fermilab. Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for the prediction of the top and bottom quark, which together form the third generation of quarks.[7]

Top quark
Top antitop quark event
A collision event involving top quarks
CompositionElementary particle
StatisticsFermionic
GenerationThird
InteractionsStrong, Weak, Electromagnetic force, Gravity
Symbol
t
AntiparticleTop antiquark (
t
)
TheorizedMakoto Kobayashi and Toshihide Maskawa (1973)
DiscoveredCDF and collaborations (1995)
Mass173.0 ± 0.4 GeV/c2[1]
Decays intobottom quark (99.8%)
strange quark (0.17%)
down quark (0.007%)
Electric charge+2/3 e
Color chargeYes
Spin1/2
Topness1
Weak isospinLH: +1/2, RH: 0
Weak hyperchargeLH: +1/3, RH: +4/3

History

In 1973, Makoto Kobayashi and Toshihide Maskawa predicted the existence of a third generation of quarks to explain observed CP violations in kaon decay.[4] The names top and bottom were introduced by Haim Harari in 1975,[8][9] to match the names of the first generation of quarks (up and down) reflecting the fact that the two were the 'up' and 'down' component of a weak isospin doublet.[10] The top quark was sometimes called truth quark in the past, but over time top quark became the predominant use.[11]

The proposal of Kobayashi and Maskawa heavily relied on the GIM mechanism put forward by Sheldon Lee Glashow, John Iliopoulos and Luciano Maiani,[12] which predicted the existence of the then still unobserved charm quark. (The other second generation quark, the strange quark, was already detected in 1968.) When in November 1974 teams at Brookhaven National Laboratory (BNL) and the Stanford Linear Accelerator Center (SLAC) simultaneously announced the discovery of the J/ψ meson, it was soon after identified as a bound state of the missing charm quark with its antiquark. This discovery allowed the GIM mechanism to become part of the Standard Model.[13] With the acceptance of the GIM mechanism, Kobayashi and Maskawa's prediction also gained in credibility. Their case was further strengthened by the discovery of the tau by Martin Lewis Perl's team at SLAC between 1974 and 1978.[14] The tau announced a third generation of leptons, breaking the new symmetry between leptons and quarks introduced by the GIM mechanism. Restoration of the symmetry implied the existence of a fifth and sixth quark.

It was in fact not long until a fifth quark, the bottom, was discovered by the E288 experiment team, led by Leon Lederman at Fermilab in 1977.[15][16][17] This strongly suggested that there must also be a sixth quark, the top, to complete the pair. It was known that this quark would be heavier than the bottom, requiring more energy to create in particle collisions, but the general expectation was that the sixth quark would soon be found. However, it took another 18 years before the existence of the top was confirmed.[18]

Early searches for the top quark at SLAC and DESY (in Hamburg) came up empty-handed. When, in the early eighties, the Super Proton Synchrotron (SPS) at CERN discovered the W boson and the Z boson, it was again felt that the discovery of the top was imminent. As the SPS gained competition from the Tevatron at Fermilab there was still no sign of the missing particle, and it was announced by the group at CERN that the top mass must be at least 41 GeV/c2. After a race between CERN and Fermilab to discover the top, the accelerator at CERN reached its limits without creating a single top, pushing the lower bound on its mass up to 77 GeV/c2.[18]

The Tevatron was (until the start of LHC operation at CERN in 2009) the only hadron collider powerful enough to produce top quarks. In order to be able to confirm a future discovery, a second detector, the DØ detector, was added to the complex (in addition to the Collider Detector at Fermilab (CDF) already present). In October 1992, the two groups found their first hint of the top, with a single creation event that appeared to contain the top. In the following years, more evidence was collected and on April 22, 1994, the CDF group submitted their paper presenting tentative evidence for the existence of a top quark with a mass of about 175 GeV/c2. In the meantime, DØ had found no more evidence than the suggestive event in 1992. A year later, on March 2, 1995, after having gathered more evidence and reanalyzed the DØ data (which had been searched for a much lighter top), the two groups jointly reported the discovery of the top at a mass of 176±18 GeV/c2.[5][6][18]

In the years leading up to the top quark discovery, it was realized that certain precision measurements of the electroweak vector boson masses and couplings are very sensitive to the value of the top quark mass. These effects become much larger for higher values of the top mass and therefore could indirectly see the top quark even if it could not be directly detected in any experiment at the time. The largest effect from the top quark mass was on the T parameter and by 1994 the precision of these indirect measurements had led to a prediction of the top quark mass to be between 145 GeV/c2 and 185 GeV/c2.[19] It is the development of techniques that ultimately allowed such precision calculations that led to Gerardus 't Hooft and Martinus Veltman winning the Nobel Prize in physics in 1999.[20][21]

Properties

  • At the final Tevatron energy of 1.96 TeV, top–antitop pairs were produced with a cross section of about 7 picobarns (pb).[22] The Standard Model prediction (at next-to-leading order with mt = 175 GeV/c2) is 6.7–7.5 pb.
  • The W bosons from top quark decays carry polarization from the parent particle, hence pose themselves as a unique probe to top polarization.
  • In the Standard Model, the top quark is predicted to have a spin quantum number of ​12 and electric charge +​23. A first measurement of the top quark charge has been published, resulting in approximately 90% confidence limit that the top quark charge is indeed +​23.[23]

Production

Because top quarks are very massive, large amounts of energy are needed to create one. The only way to achieve such high energies is through high energy collisions. These occur naturally in the Earth's upper atmosphere as cosmic rays collide with particles in the air, or can be created in a particle accelerator. In 2011, after the Tevatron ceased operations, the Large Hadron Collider at CERN became the only accelerator that generates a beam of sufficient energy to produce top quarks, with a center-of-mass energy of 7 TeV. There are multiple processes that can lead to the production of top quarks, but they can be conceptually divided in two categories.

Top-quark pairs

Ttbar production via gg fusion
gluon-gluon fusion
Ttbar production (t channel)
t-channel
Ttbar production via qqbar annihilation
quark-antiquark annihilation

The most common is production of a top–antitop pair via strong interactions. In a collision, a highly energetic gluon is created, which subsequently decays into a top and antitop. This process was responsible for the majority of the top events at Tevatron and was the process observed when the top was first discovered in 1995.[24] It is also possible to produce pairs of top–antitop through the decay of an intermediate photon or Z-boson. However, these processes are predicted to be much rarer and have a virtually identical experimental signature in a hadron collider like Tevatron.

Single top quarks

Single-top production (s channel)
s-channel
Single-top production (t-channel)
t-channel
Single top production (tW channel)
tW channel

A distinctly different process is the production of single top quarks via weak interaction. This can happen in several ways (called channels): either an intermediate W-boson decays into a top and antibottom quark ("s-channel") or a bottom quark (probably created in a pair through the decay of a gluon) transforms to a top quark by exchanging a W-boson with an up or down quark ("t-channel"). A single top quark can also be produced in association with a W boson, requiring an initial state bottom quark ("tW-channel"). The first evidence for these processes was published by the DØ collaboration in December 2006,[25] and in March 2009 the CDF[26] and DØ[24] collaborations released twin papers with the definitive observation of these processes. The main significance of measuring these production processes is that their frequency is directly proportional to the | Vtb |2 component of the CKM matrix.

Decay

Ttbar decay channels
All possible final states of the decay of a top-quark pair

Because of its enormous mass, the top quark is extremely short-lived with a predicted lifetime of only 5×10−25 s.[3] As a result, top quarks do not have time before they decay to form hadrons as other quarks do, which provides physicists with the unique opportunity to study the behavior of a "bare" quark. The only known way the top quark can decay is through the weak interaction producing a W-boson and a bottom-type quark.

In particular, it is possible to directly determine the branching ratio Γ(W+b) / Γ(W+q (q = b,s,d)). The best current determination of this ratio is 0.91±0.04.[27] Since this ratio is equal to | Vtb |2 according to the Standard Model, this gives another way of determining the CKM element | Vtb |, or in combination with the determination of | Vtb | from single top production provides tests for the assumption that the CKM matrix is unitary.[28]

The Standard Model also allows more exotic decays, but only at one loop level, meaning that they are extremely suppressed. In particular, it is possible for a top quark to decay into another up-type quark (an up or a charm) by emitting a photon or a Z-boson.[29] Searches for these exotic decay modes have provided no evidence for their existence in accordance with expectations from the Standard Model. The branching ratios for these decays have been determined to be less than 5.9 in 1,000 for photonic decay and less than 2.1 in 1,000 for Z-boson decay at 95% confidence.[27]

Mass and coupling to the Higgs boson

The Standard Model describes fermion masses through the Higgs mechanism. The Higgs boson has a Yukawa coupling to the left- and right-handed top quarks. After electroweak symmetry breaking (when the Higgs acquires a vacuum expectation value), the left- and right-handed components mix, becoming a mass term.

The top quark Yukawa coupling has a value of

where v = 246 GeV is the value of the Higgs vacuum expectation value.

Yukawa couplings

In the Standard Model, all of the quark and lepton Yukawa couplings are small compared to the top quark Yukawa coupling. Understanding this hierarchy in the fermion masses is an open problem in theoretical physics. Yukawa couplings are not constants and their values change depending on the energy scale (distance scale) at which they are measured. The dynamics of Yukawa couplings are determined by the renormalization group equation.

One of the prevailing views in particle physics is that the size of the top quark Yukawa coupling is determined by the renormalization group, leading to the "quasi-infrared fixed point."

The Yukawa couplings of the up, down, charm, strange and bottom quarks, are hypothesized to have small values at the extremely high energy scale of grand unification, 1015 GeV. They increase in value at lower energy scales, at which the quark masses are generated by the Higgs. The slight growth is due to corrections from the QCD coupling. The corrections from the Yukawa couplings are negligible for the lower mass quarks.

If, however, a quark Yukawa coupling has a large value at very high energies, its Yukawa corrections will evolve and cancel against the QCD corrections. This is known as a (quasi-) infrared fixed point. No matter what the initial starting value of the coupling is, if it is sufficiently large it will reach this fixed point value. The corresponding quark mass is then predicted.

The top quark Yukawa coupling lies very near the infrared fixed point of the Standard Model. The renormalization group equation is:

where g3 is the color gauge coupling, g2 is the weak isospin gauge coupling, and g1 is the weak hypercharge gauge coupling. This equation describes how the Yukawa coupling changes with energy scale μ. Solutions to this equation for large initial values yt cause the right-hand side of the equation to quickly approach zero, locking yt to the QCD coupling g3. The value of the fixed point is fairly precisely determined in the Standard Model, leading to a top quark mass of 230 GeV. However, if there is more than one Higgs doublet, the mass value will be reduced by Higgs mixing angle effects in an unpredicted way.

In the minimal supersymmetric extension of the Standard Model (MSSM), there are two Higgs doublets and the renormalization group equation for the top quark Yukawa coupling is slightly modified:

where yb is the bottom quark Yukawa coupling. This leads to a fixed point where the top mass is smaller, 170–200 GeV. The uncertainty in this prediction arises because the bottom quark Yukawa coupling can be amplified in the MSSM. Some theorists believe this is supporting evidence for the MSSM.

The quasi-infrared fixed point has subsequently formed the basis of top quark condensation theories of electroweak symmetry breaking in which the Higgs boson is composite at extremely short distance scales, composed of a pair of top and antitop quarks.

See also

References

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Further reading

External links

Ann Heinson

Ann Heinson is an American high-energy particle physicist known for her work on single top quark physics. She established and lead the DØ Single Top Group which first published experimental observations of the top quark, and in 1997 she co-authored a paper which laid the foundations for further investigation into the top quark.

Bottom quark

The bottom quark or b quark, also known as the beauty quark, is a third-generation quark with a charge of −1/3 e.

All quarks are described in a similar way by electroweak and quantum chromodynamics, but the bottom quark has exceptionally low rates of transition to lower-mass quarks. The bottom quark is also notable because it is a product in almost all top quark decays, and is a frequent decay product of the Higgs boson.

Christopher T. Hill

Christopher T. Hill (born June 19, 1951) is an American theoretical physicist at the Fermi National Accelerator Laboratory who did undergraduate work in physics at M.I.T. (B.S., M.S., 1972), and graduate work at Caltech (Ph.D., 1977, Murray Gell-Mann). Hill's Ph.D. thesis, "Higgs Scalars and the Nonleptonic Weak Interactions" (1977) contains the first discussion of the two-Higgs-doublet model.

Hill has made numerous contributions to dynamical theories of electroweak symmetry breaking, and is an originator of the top quark infrared fixed point, top quark condensates, topcolor, and dimensional deconstruction. He has coauthored an extensive review of strong dynamical theories.

Hill is an originator of cosmological models of dark energy and dark matter based upon ultra-low mass bosons associated with neutrino masses and was first to propose that the cosmological constant is connected to the neutrino mass, as . He has also developed modern theories of the origin of ultra-high-energy nucleons and neutrinos from grand unification relics, such as cosmic strings.

He has more recently focused on the idea that all fundamental mass scales may be associated with spontaneously broken scale symmetry, or (Weyl symmetry). With coauthors, he has identified a new phenomenon, dubbed "inertial symmetry breaking," by which the scale of gravity (Planck mass) and the inflationary phase of the ultra-early universe are generated together as part of a unified phenomenon.

Hill is a "Distinguished Scientist" at Fermilab, former Head of the Theoretical Physics Department (2005 - 2012) and a Fellow of the American Physical Society. He has co-authored three popular books with Nobel laureate Leon Lederman about physics and cosmology, and the commissioning of the Large Hadron Collider.

Collider Detector at Fermilab

The Collider Detector at Fermilab (CDF) experimental collaboration studies high energy particle collisions from the Tevatron, the world's former highest-energy particle accelerator. The goal is to discover the identity and properties of the particles that make up the universe and to understand the forces and interactions between those particles.

CDF is an international collaboration of about 600 physicists (from about 30 American universities and National laboratories and about 30 groups from universities and national laboratories from Italy, Japan, UK, Canada, Germany, Spain, Russia, Finland, France, Taiwan, Korea, and Switzerland). The CDF detector itself weighed 5000 tons [1] and was about 12 meters in all three dimensions. The goal of the experiment is to measure exceptional events out of the billions of particle collisions in order to:

Look for evidence for phenomena beyond the Standard Model of particle physics

Measure and study the production and decay of heavy particles such as the Top and Bottom Quarks, and the W and Z bosons

Measure and study the production of high-energy particle jets and photons

Study other phenomena such as diffractionThe Tevatron collided protons and antiprotons at a center-of-mass energy of about 2 TeV. The very high energy available for these collisions made it possible to produce heavy particles such as the Top quark and the W and Z bosons, which weigh much more than a proton (or antiproton). These heavier particles were identified through their characteristic decays. The CDF apparatus recorded the trajectories and energies of electrons, photons and light hadrons. Neutrinos did not register in the apparatus which led to an apparent missing energy. Other hypothetical particles might leave a missing energy signature, and some searches for new phenomena are based on that.

There is another experiment similar to CDF called D0 which had a detector located at another point on the Tevatron ring.

Compact Linear Collider

The Compact Linear Collider (CLIC) is a concept for a future linear particle accelerator that aims to explore the next energy frontier. CLIC would collide electrons with positrons and is currently the only mature option for a multi-TeV linear collider. The accelerator would be between 11 and 50 km (7 and 31 mi) long, more than ten times longer than the existing Stanford Linear Accelerator (SLAC) in California, USA. CLIC is proposed to be built at CERN, across the border between France and Switzerland near Geneva, with first beams starting by the time the Large Hadron Collider (LHC) has finished operations around 2035.The CLIC accelerator would use a novel two-beam acceleration technique at an acceleration gradient of 100 MV/m, and its staged construction would provide collisions at three centre-of-mass energies up to 3 TeV for optimal physics reach. Research and development (R&D) are being carried out in the study to achieve the high precision physics goals under challenging beam and background conditions.

CLIC aims to discover new physics beyond the Standard Model of particle physics, through precision measurements of Standard Model properties as well as direct detection of new particles. The collider would offer high sensitivity to electroweak states, exceeding the predicted precision of the full LHC programme. The current CLIC design includes the possibility for electron beam polarisation, further constraining the underlying physics.The CLIC study produced a Conceptual Design Report (CDR) in 2012 and is working to present the case for the CLIC concept for the next Update of the European Strategy for Particle Physics in 2019-2020.

D0 experiment

The DØ experiment (sometimes written D0 experiment, or DZero experiment) consists of a worldwide collaboration of scientists conducting research on the fundamental nature of matter. DØ was one of two major experiments (the other was the CDF experiment) located at the Tevatron Collider at Fermilab in Batavia, Illinois, USA. The Tevatron was the world's highest-energy accelerator from 1983 until 2009, when its energy was surpassed by the Large Hadron Collider. The DØ experiment stopped taking data in 2011, when the Tevatron shut down, but data analysis is still ongoing. The DØ detector is preserved in Fermilab's DØ Assembly Building as part of a historical exhibit for public tours.DØ research is focused on precise studies of interactions of protons and antiprotons at the highest available energies. These collisions result in "events" containing many new particles created through the transformation of energy into mass according to the relation E=mc2. The research involves an intense search for subatomic clues that reveal the character of the building blocks of the universe.

Eta meson

The eta (η) and eta prime meson (η′) are isosinglet mesons made of a mixture of up, down and strange quarks and their antiquarks. The charmed eta meson (ηc) and bottom eta meson (ηb) are similar forms of quarkonium; they have the same spin and parity as the (light) η defined, but are made of charm quarks and bottom quarks respectively. The top quark is too heavy to form a similar meson, due to its very fast decay.

Hadronization

In particle physics, hadronization (or hadronisation) is the process of the formation of hadrons out of quarks and gluons. This occurs after high-energy collisions in a particle collider in which quarks or gluons are created. Due to colour confinement, these cannot exist individually. In the Standard Model they combine with quarks and antiquarks spontaneously created from the vacuum to form hadrons. The QCD (Quantum Chromodynamics) of the hadronization process are not yet fully understood, but are modeled and parameterized in a number of phenomenological studies, including the Lund string model and in various long-range QCD approximation schemes.The tight cone of particles created by the hadronization of a single quark is called a jet. In particle detectors, jets are observed rather than quarks, whose existence must be inferred. The models and approximation schemes and their predicted jet hadronization, or fragmentation, have been extensively compared with measurement in a number of high energy particle physics experiments, e.g. TASSO, OPAL and H1.Hadronization also occurred shortly after the Big Bang when the quark–gluon plasma cooled to the temperature below which free quarks and gluons cannot exist (about 170 MeV). The quarks and gluons then combined into hadrons.

A top quark, however, has a mean lifetime of 5×10−25 seconds, which is shorter than the time scale at which the strong force of QCD acts, so a top quark decays before it can hadronize, allowing physicists to observe a "bare quark." Thus, they have not been observed as components of any observed hadron, while all other quarks have been observed only as components of hadrons.

Hyperon

In particle physics, a hyperon is any baryon containing one or more strange quarks, but no charm, bottom, or top quark. This form of matter may exist in a stable form within the core of some neutron stars.

Infrared fixed point

In physics, an infrared fixed point is a set of coupling constants, or other parameters that evolve from initial values at very high energies (short distance), to fixed stable values, usually predictable, at low energies (large distance). This usually involves the use of the renormalization group, which specifically details the way parameters in a physical system (a quantum field theory) depend on the energy scale being probed.

Conversely, if the length-scale decreases and the physical parameters approach fixed values, then we have ultraviolet fixed points. The fixed points are generally independent of the initial values of the parameters over a large range of the initial values. This is known as universality.

Lambda baryon

The Lambda baryons are a family of subatomic hadron particles containing one up quark, one down quark, and a third quark from a higher flavour generation, in a combination where the quantum wave function changes sign upon the flavour of any two quarks being swapped (thus differing from a Sigma baryon). They are thus baryons, with total isospin of 0, and have either neutral electric charge or the elementary charge +1.

Lambda baryons are usually represented by the symbols Λ0, Λ+c, Λ0b, and Λ+t. In this notation, the superscript character indicates whether the particle is electrically neutral (0) or carries a positive charge (+). The subscript character, or its absence, indicates whether the third quark is a strange quark (Λ0) (no subscript), a charm quark (Λ+c), a bottom quark (Λ0b), or a top quark (Λ+t). Physicists do not expect to observe a Lambda baryon with a top quark because the Standard Model of particle physics predicts that the mean lifetime of top quarks is roughly 5×10−25 seconds; that is about 1/20 of the mean timescale for strong interactions, which indicates that the top quark would decay before a Lambda baryon could form a hadron.

Quark

A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within hadrons, which include baryons (such as protons and neutrons) and mesons. For this reason, much of what is known about quarks has been drawn from observations of hadrons.

Quarks have various intrinsic properties, including electric charge, mass, color charge, and spin. They are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction), as well as the only known particles whose electric charges are not integer multiples of the elementary charge.

There are six types, known as flavors, of quarks: up, down, strange, charm, bottom, and top. Up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay: the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in the universe, whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators). For every quark flavor there is a corresponding type of antiparticle, known as an antiquark, that differs from the quark only in that some of its properties (such as the electric charge) have equal magnitude but opposite sign.

The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until deep inelastic scattering experiments at the Stanford Linear Accelerator Center in 1968. Accelerator experiments have provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered.

Stop squark

In particle physics, a stop squark, symbol t͂, is the superpartner of the top quark as predicted by supersymmetry (SUSY). It is a sfermion, which means it is a spin-0 boson (scalar boson). While the top quark is the heaviest known quark, the stop squark is actually often the lightest squark in many supersymmetry models.The stop squark is a key ingredient of a wide range of SUSY models that address the hierarchy problem of the Standard Model (SM) in a natural way. A boson partner to the top quark would stabilize the Higgs boson mass against quadratically divergent quantum corrections, provided its mass is close to the electroweak symmetry breaking energy scale. If this was the case then the stop squark would be accessible at the Large Hadron Collider. In the generic R-parity conserving Minimal Supersymmetric Standard Model (MSSM) the scalar partners of right-handed and left-handed top quarks mix to form two stop mass eigenstates. Depending on the specific details of the SUSY model and the mass hierarchy of the sparticles, the stop might decay into a bottom quark and a chargino, with a subsequent decay of the chargino into the lightest neutralino (which is often the lightest supersymmetric particle).

Many searches for evidence of the stop squark have been performed by both the ATLAS and CMS experiments at the LHC but so far no signal has been discovered. In January 2019, the CMS Collaboration published findings excluding stop squarks with masses as large as 1230 GeV at 95% confidence level.

T meson

T mesons are hypothetical mesons composed of a top quark and either an up (T0), down (T+), strange (T+s) or charm antiquark (T0c). Because of the top quark's short lifetime, T mesons are not expected to be found in nature. The combination of a top quark and top antiquark is not a T meson, but rather toponium. Each T meson has an antiparticle that is composed of a top antiquark and an up (T0), down (T−), strange (T−s) or charm quark (T0c) respectively.

Theta meson

The theta meson (θ) is a hypothetical form of quarkonium (i.e. a flavourless meson) formed by a top quark (t) and top antiquark (t). As a P-odd and C-odd tt state, it is analogous to the ϕ (ss), J/ψ (cc) and ϒ (bb) mesons. Due to the top quark's short lifetime, the theta meson is not expected to be observed in nature.

Top quark condensate

In particle physics, the top quark condensate theory (or top condensation) is an alternative to the Standard Model fundamental Higgs field, where the Higgs boson is a composite field, composed of the top quark and its antiquark. These are bound together by a new force called topcolor, analogous to the binding of Cooper pairs in a BCS superconductor, or mesons in the strong interactions. The idea of binding of top quarks is motivated because it is comparatively heavy, with a measured mass is approximately 173 GeV (comparable to the electroweak scale), and so its Yukawa coupling is of order unity, suggesting the possibility of strong coupling dynamics. at higher energy scales.

This model attempts to explain how the electroweak scale may match the

top quark mass.

Top quark condensation is based upon the "infrared fixed point" for the top quark Higgs-Yukawa coupling, proposed in 1981 by Hill,

based upon an earlier proposal of Pendleton and Ross.

The infrared fixed point surprisingly predicted that the top

quark would be heavy, contrary to the prevailing view of the early 1980's. Indeed,

the top quark was discovered in 1995 at the large mass of 173 GeV.

The infrared-fixed point implies that it

is strongly coupled to the Higgs boson at very high energies, corresponding

to the Landau pole of the Higgs-Yukawa coupling. At this high scale the boundstate Higgs forms, and the coupling relaxes in the infrared to its measured value of order unity

by the renormalization group.

The idea in its present form was described by Yoichiro Nambu and subsequently

by Miransky, Tanabashi, and Yamawaki

and Bardeen, Hill and Lindner,

who connected the theory to the renormalization group and improved its predictions.

The simplest top condensation models predicted that the Higgs boson mass would be larger than the observed 173 GeV top quark mass, and have now been ruled out by the LHC discovery of the Higgs boson at a mass scale of 125 GeV.

However, extended versions introducing more particles can be made consistent with the observed top quark mass. The general idea of a composite Higgs boson, connected

in a fundamental way to the top quark, remains compelling, though the full details are

not yet understood.

A composite Higgs boson arises naturally in Topcolor models, that are extensions of the standard model in analogy to quantum chromodynamics. To be natural, without excessive fine-tuning (i.e. to stabilize the Higgs mass from large radiative corrections), the theory requires new physics at a relatively low energy scale. Placing new physics at 10 TeV, for instance, the model predicts the top quark to be significantly heavier than observed (at about 600 GeV vs. 171 GeV). "Top Seesaw" models, also based upon Topcolor, circumvent this difficulty.

Topcolor

In theoretical physics, topcolor is a model of dynamical electroweak symmetry breaking in which the top quark and anti-top quark form a composite Higgs boson by a new force arising from massive "top gluons." This is analogous to the phenomenon of superconductivity where Cooper pairs are

formed by the exchange of phonons. The pairing dynamics and its solution was

treated in the Bardeen-Hill-Lindner model.

The solution to composite Higgs models was actually anticipated

in 1981, and found to be the Infrared fixed point for the top quark mass.

Recently this has been revisited in the context of "Scalar Democracy" in which a rich system of composite Higgs bosons may exist at very high energies at which all

fermion pairs in the standard model form composite scalars by gravity,

leading to a rich spectrum of heavy Higgs bosons.Topcolor naturally involves an extension of the standard model color gauge group to a product group SU(3)xSU(3)xSU(3)x... One of the gauge groups contains

the top and bottom quarks, and has a sufficiently large coupling constant to cause the condensate to form. The topcolor model anticipates the idea of dimensional deconstruction and extra space dimensions, as well as the large mass of the top quark.

UA1 experiment

The UA1 experiment (an abbreviation of Underground Area 1) was a high-energy physics experiment that ran at CERN's Proton-Antiproton Collider (SppS), a modification of the one-beam Super Proton Synchrotron (SPS). The data was recorded between 1981 and 1990. The joint discovery of the W and Z bosons by this experiment and the UA2 experiment in 1983 led to the Nobel Prize for physics being awarded to Carlo Rubbia and Simon van der Meer in 1984. Peter Kalmus and John Dowell, from the UK groups working on the project, were jointly awarded the 1988 Rutherford Medal and Prize from the Institute of Physics for their outstanding roles in the discovery of the W and Z particles.

It was named as the first experiment in a CERN "Underground Area" (UA), i.e. located underground, outside of the two main CERN sites, at an interaction point on the SPS accelerator, which had been modified to operate as a collider.

The UA1 central detector was crucial to understanding the complex topology of proton-antiproton collisions. It played a most important role in identifying a handful of W and Z particles among billions of collisions.

After the discovery of the W and Z boson, the UA1 collaboration went on to search for the top quark. Physicists had anticipated its existence since 1977, when its partner — the bottom quark — was discovered. It was felt that the discovery of the top quark was imminent. In June 1984, Carlo Rubbia at the UA1 experiment expressed to the New York Times that evidence of the top quark "looks really good". Over the next months it became clear that UA1 had overlooked a significant source of background. The top quark was ultimately discovered in 1994–1995 by physicists at Fermilab with a mass near 175 GeV.

The UA1 was a huge and complex detector for its day. It was designed as a general-purpose detector.

The detector was a 6-chamber cylindrical assembly 5.8 m long and 2.3 m in diameter, the largest imaging drift chamber of its day. It recorded the tracks of charged particles curving in a 0.7 Tesla magnetic field, measuring their momentum, the sign of their electric charge and their rate of energy loss (dE/dx). Atoms in the argon-ethane gas mixture filling the chambers were ionised by the passage of charged particles. The electrons which were released drifted along an electric field shaped by field wires and were collected on sense wires. The geometrical arrangement of the 17000 field wires and 6125 sense wires allowed a spectacular 3-D interactive display of reconstructed physics events to be produced.The UA1 detector was conceived and designed in 1978/9, with the proposal submitted in mid-1978.Since the end of running, the magnet used in the UA1 experiment has been used for other high energy physics experiments, notably the NOMAD and T2K neutrino experiments.

UA2 experiment

The Underground Area 2 (UA2) experiment was a high-energy physics experiment at the Proton-Antiproton Collider (SppS) — a modification of the Super Proton Synchrotron (SPS) — at CERN. The experiment ran from 1981 until 1990, and its main objective was to discover the W and Z bosons. UA2, together with the UA1 experiment, succeeded in discovering these particles in 1983, leading to the 1984 Nobel Prize in Physics being awarded to Carlo Rubbia and Simon van der Meer. The UA2 experiment also observed the first evidence for jet production in hadron collisions in 1981, and was involved in the searches of the top quark and of supersymmetric particles. Pierre Darriulat was the spokesperson of UA2 from 1981 to 1986, followed by Luigi Di Lella from 1986 to 1990.

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